Apparatus and method for controlling an electric heating assembly

ABSTRACT

In apparatus and a method for providing electronic control of an electric heating assembly, a radiant electric heater ( 10, 110 ) is arranged at a lower surface ( 22, 124 ) of a glass-ceramic cooking plate ( 12, 112 ), the plate having an upper surface ( 40, 138 ) for receiving a cooking vessel ( 42, 136 A,  136 B). A temperature sensor ( 24, 140 ) monitors temperature at or adjacent to the cooking plate ( 12, 112 ) and provides an electrical output as a function of temperature. Control means ( 30, 142 ) connected to the temperature sensor ( 24, 140 ) and to the heater ( 10, 110 ) controls energising of the heater from a power supply ( 28, 134 ) for energising the heater at a plurality of user selectable power levels including a full power level. When the heater ( 10, 110 ) is energised at the full power level it is energised to heat the cooking plate ( 12, 112 ) to a first temperature level during a predetermined initial period (A) of 20 to 50 minutes and is thereafter (C) energised to heat the cooking plate to a second temperature level, lower than the first temperature level.

This invention relates to an apparatus and a method for controlling anelectric heating assembly in which a radiant electric heater is arrangedbeneath a glass-ceramic cooking plate in a cooking appliance.

When a radiant electric heater is operating beneath a glass-ceramiccooking plate, in order to heat a cooking vessel located on an uppersurface of the cooking plate, the lower surface of the cooking plate isheated by direct radiation from the heater and heat is transferredthrough the cooking plate to the cooking vessel on the upper surface. Infree radiation conditions, that is without any cooking vessel on thecooking plate, the radiant heaters in a glass-ceramic cooktop appliancewill transmit heat to a back wall, for example a wall of a kitchen, andto any side wall adjacent to the cooktop. Temperature limits for theback wall and any side walls are specified in European Safety StandardEN60335.

Further, in order to prevent thermal damage occurring to the cookingplate, the temperature, particularly of the lower surface, must becontrolled. In order to control the temperature of the lower surface ofthe glass-ceramic cooking plate, temperature limiters are provided inheaters to de-energise the heaters at a predetermined temperature. Suchlimiters, which have generally been of electro-mechanical construction,are set to respond to the temperature of the upper surface of thecooking plate.

As a precaution, in order to meet the various requirements of theglass-ceramic cooktop and appropriate safety standards, the temperaturelimiter is generally set to switch, in free radiation conditions, at arelatively low temperature of the upper surface (commonly referred to astop glass temperature), which may be less than 550 degrees Celsius. Suchan arrangement is unsatisfactory as it means that the rate of heattransfer, particularly to cooking vessels having less than ideal contactwith the upper surface of the cooking plate, is reduced by prematureswitching of the limiter, making it impossible to make maximum andoptimum use of the available power of the heaters.

It is known from EP-A-0 886 459 to provide an initial temperature boostsuch that the temperature of a glass ceramic cooking plate exceeds apredetermined continuous safe level. This is balanced by subsequentlyreducing the temperature such that in the longer term the continuoussafe temperature is not exceeded. The initial boost is relatively short,about 10 minutes, and the subsequent temperature reduction is topreserve the life of the glass ceramic cooktop, not to satisfy back walland side wall temperature requirements.

It is an object of the present invention to overcome or minimise theabove problem.

According to one aspect of the present invention there is providedapparatus for providing electronic control of an electric heatingassembly in which a radiant electric heater is arranged at a lowersurface of a glass-ceramic cooking plate, the cooking plate having anupper surface for receiving a cooking vessel, the apparatus comprising:a temperature sensor for monitoring temperature at or adjacent to thecooking plate, which sensor provides an electrical output as a functionof temperature; and control means connected to the temperature sensorand to the heater, for controlling energising of the heater from a powersupply, the control means being adapted and arranged to energise theheater at a plurality of user selectable power levels including a fullpower level, wherein when the heater is energised at the full powerlevel it is energised to heat the cooking plate to a first temperaturelevel for a predetermined initial period of 20 to 50 minutes and isthereafter energised at a second temperature level, lower than the firsttemperature level.

According to a further aspect of the present invention there is provideda method of providing electronic control of an electric heating assemblyin which a radiant electric heater is arranged at a lower surface of aglass-ceramic cooking plate, the cooking plate having an upper surfacefor receiving a cooking vessel, the method comprising: providing atemperature sensor for monitoring temperature at or adjacent to thecooking plate, which sensor provides an electrical output as a functionof temperature; and providing control means connected to the temperaturesensor and to the heater, for controlling energising of the heater froma power supply, the control means being adapted and arranged to energisethe heater at a plurality of user selectable power levels including afull power level, wherein when the heater is energised at the full powerlevel it is energised to heat the cooking plate to a first temperaturelevel during a predetermined initial period of 20 to 50 minutes and isthereafter energised at a second temperature level, lower than the firsttemperature level.

During an initial minor proportion of the predetermined initial periodthe heater may be energised at a boost power level, in excess of thefirst power level.

The second temperature level may be between about 75 percent and about85 percent, preferably about 83 percent, of the first temperature level.

The length of the predetermined initial period may be dependent on thetime elapsed since the control means was last at full power. The lengthof the predetermined initial period may be inversely proportional to thetime elapsed since the control means was last at the full power level.

Reduction from the first temperature level to the second temperaturelevel may be effected in a continuous or stepwise manner. If stepwise itmay be effected in a single step or in a plurality of steps.

The control means may comprise a microprocessor-based controller intowhich the predetermined initial period and a setting for the secondtemperature level are permanently programmed for automaticimplementation.

The temperature sensor may provide an electrical output as a function oftemperature of the upper surface of the glass-ceramic cooking plate.

The temperature sensor may comprise a device whose electrical resistancechanges as a function of temperature and may comprise a platinumresistance temperature detector.

The temperature sensor may be provided on, or spaced behind, the lowersurface of the glass-ceramic cooking plate.

The heater may have a main heating zone at least partially surrounded byat least one additional heating zone, the main heating zone beingenergisable alone or together with the at least one additional heatingzone. The at least one additional heating zone may be arranged againstat least one side of the main heating zone, for example at oppositesides thereof. The predetermined initial time may be about 20 minutes toabout 40 minutes when the main heating zone is energised together withthe at least one additional heating zone, and may be about 30 minutes toabout 50 minutes when the main heating zone is energised alone.

Alternatively, in particular where only a single heating zone isprovided the predetermined initial time may be about 20 minutes to about40 minutes.

The present invention enables full available power of a radiant heaterto be applied for the maximum period of time, without the specifiedlimit temperature for EN60335 being exceeded.

The settings for the predetermined initial period and the secondtemperature level are determined by experiment during manufacture, foreach specific heater assembly, and fixedly programmed into the controlmeans during the manufacturing process.

For a better understanding of the invention and to show more clearly howit may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

FIG. 1 is a diagrammatic perspective view showing a glass-ceramiccooktop appliance mounted adjacent to a back wall and a side wall;

FIG. 2 is a plan view of one embodiment of an electric heater assemblyadapted for control according to the present invention;

FIG. 3 is a section along line A-A of the heater of the assembly of FIG.2;

FIG. 4 is a graphical illustration of temperature of the upper surfaceof a cooking plate in the heating assembly of FIGS. 2 and 3, duringcontrol according to the present invention;

FIG. 5 is a graphical illustration of power levels supplied to a heaterduring operation of the cooking assembly of FIGS. 2 and 3;

FIG. 6 is a plan view of another embodiment of an electric heaterassembly adapted for control according to the present invention;

FIG. 7 is a cross-sectional view of the heater of the assembly of FIG.6;

FIG. 8 is a graphical illustration of temperature of the upper surfaceof a cooking plate in the heating assemblies of FIGS. 6 and 7, duringcontrol according to the present invention; and

FIGS. 9 and 10 are plan views of alternative embodiments of electricheaters for use in an assembly for control according to the presentinvention.

Referring to FIG. 1, there is shown a glass-ceramic cooktop appliance 2mounted on a counter surface 4 adjacent to a back wall 6 and a side wall8.

Referring to FIGS. 2 and 3, an electric heater 10 is arranged beneath aglass-ceramic cooking plate 12 in the cooking appliance 2. The heater 10comprises a metal dish 14 having a base layer 16 of thermal insulationmaterial, such as microporous thermal insulation material. A heatingelement 18 is supported on the base layer 16. As shown, the heatingelement 18 comprises a corrugated metal ribbon supported edgewise on thebase layer 16. However, the heating element 18 could comprise otherforms, such as wire or foil, or one or more infrared lamps. Any of thewell-known forms of heating element, or combinations thereof, could beconsidered.

A peripheral wall 20 of thermal insulation material is provided, a topsurface of which contacts a lower surface 22 of the glass-ceramiccooking plate 12.

A temperature sensor 24 is arranged to extend partially across theheater, between the heating element 18 and the cooking plate 12. Thetemperature sensor 24 comprises a tube containing a device whichprovides an electrical output as a function of temperature or a beam orother member carrying a device which provides an electrical output as afunction of temperature. Such device may have an electrical parameter,such as electrical resistance, which changes as a function oftemperature. In particular, the device comprises a platinum resistancetemperature detector.

As an alternative to the temperature sensor 24, a temperature sensorcould be provided deposited on, or secured in contact with, the lowersurface 22 of the cooking plate 12.

A terminal block 26 is arranged at the edge of the heater and by meansof which the heating element 18 is electrically connected to a powersupply 28 for energising.

Control circuitry 30 is provided for the heater 10. Such controlcircuitry comprises a microcontroller 32, which is amicroprocessor-based control unit. An energy regulator 34 is alsoprovided, which has a control knob 36 by means of which a plurality ofuser-selectable energy (power level) settings of the heater 10,including a full power setting, can be achieved in known manner.

Power is supplied to the heater 10 from the power supply 28 by way of arelay 38, or by way of a solid state switch means, such as a triac.

The temperature sensor 24 is calibrated in association with themicrocontroller 32 to provide an electrical output which is tuned as afunction of temperature of an upper surface 40 of the cooking plate 12,which upper surface 40 is arranged to receive a cooking vessel 42.

The temperature of the glass-ceramic cooking plate 12 must not exceed acertain level in order to prevent thermal damage to the glass-ceramicmaterial. For optimum cooking performance, it is required to be able toheat up the cooking vessel 42 and its contents as rapidly as possible,for example to achieve rapid boiling of the contents of the cookingvessel 42. Accordingly, it is desirable for the temperature of the uppersurface 40 of the cooking plate 12, at which the temperature sensor 24operates for controlling the heater 18, to be as high as permissible.However, this must not be such as to result in an unacceptably hightemperature of the cooking plate 12, or an unacceptably high temperatureof the back wall a limit for which is specified in European SafetyStandard EN60335.

In the present invention it has been found that for a heater 10 operatedin a free radiation condition at a full power level setting andcontrolled by way of the temperature sensor 24, such conditions can besafely maintained with the cooking plate at a first temperature levelfor a predetermined initial period without the temperature of the backwall 6 and side wall 8 exceeding the specified limit. It has been foundthat such predetermined initial period is from about 20 to about 40minutes and is typically about 30 minutes. It has also been found thatif, at the end of such predetermined initial period, the power level ofthe heater 10 is then reduced such that the temperature of the cookingplate is reduced from the first temperature level to a secondtemperature level which is between about 75 percent and 85 percent,preferably about 83 percent, of the first temperature level(corresponding to a power level of about 60 percent to about 70 percentof the power level corresponding to the first temperature level), thetemperature of the back wall 6 and side wall 8 is maintained at a levelwhich does not exceed the specified limit. The microcontroller 32 isprogrammed in the factory, during manufacture of the heater 10 and itsassociated control circuitry, with the necessary data for thepredetermined initial period and the reduced temperature level. Suchprogrammed data is thereafter automatically implemented by themicrocontroller 32 to control the heater 10.

The controlling operation is illustrated in FIG. 4, which is a plot ofthe temperature TE in degrees Celsius of the upper surface 40 of thecooking plate 12 (known as the top glass temperature) against time TI inminutes at the full power setting. During a pre-set initial period A of30 minutes, the heater 10 is operated at a boost power level for aperiod B of about 7 minutes, followed by operation at a normal fullpower level for a further 23 minutes. During the boost period, thetemperature of the upper surface 40 of the cooking plate 12 exceeds 600degrees Celsius and during the remainder of the predetermined initialperiod the temperature of the upper surface 40 of the cooking plate 12is maintained at around 600 degrees Celsius. This enables rapid heatingto boiling to take place in the cooking vessel 42. However, during thisinitial period A the temperature of the back wall 6 and side wall 8 doesnot exceed the limit specified by European Safety Standard EN60335. Atthe end of the period A, the microcontroller 32 automatically reducesthe power level of the heater 10 to a lower fallback level such that thetemperature of the cooking plate reduces to a second temperature whichis about 75 to 85 percent, preferably about 83 percent, of the previous(first) temperature level. Such reduction, as denoted by referencenumeral 44, can be effected in one or more steps, or continuously.During the subsequent ongoing period C, the temperature of the uppersurface 40 of the cooking plate 12 is maintained at about 500 degreesCelsius and this ensures that the back wall 6 and side wall 8 aremaintained at a temperature which does not exceed the specified limit.However, as shown in FIG. 4, the reduced temperature level is not suchas to interfere with a temperature band 46, required for fryingactivities, and a temperature band 48, required for continuousboiling/simmering activities.

During normal operation, the heater 10 may be switched off, or to alower power level setting, by a user and then back to full power whilethe temperature of the back wall 6 and side wall 8 is still elevated. Inthis case the fallback (second) temperature level requires to bere-introduced in a short time compared with the situation when theheater is first energised. In this case, the time at full (first) power(i.e., first temperature), originally set to full power, may be reducedby an amount inversely proportional to the time interval since theheater was last at full power.

Thus, for example, the time before the heater is operated at thefallback temperature level may be the initial time (e.g., 30 minutes)less half the time interval since the heater was last at full power. Asa practical example, as illustrated in FIG. 5, the heater is switched tofull power, and reverts to the fallback temperature level after 30minutes as shown by point E. The heater is then switched off, or to lowpower, at 40 minutes as represented by point F and is subsequentlyswitched back to full power at 50 minutes as represented by point G. Inthis case, the heater remains at full power for (50−30)/2 minutes, i.e.10 minutes, before reverting to the fallback temperature level asrepresented by point H.

In more detail, after the heater is switched to full power from cold,for example to boil a pan of water, the power level is set by thecontrol circuitry at the boost power level for a period of 7 minutes toprovide accelerated initial heat up. At point D, the power level isreduced to normal full power, that is to the first temperature. At pointE, that is after a total of 30 minutes of boost and full power, thetemperature level reverts to the fallback (second) temperature level. Atthis temperature level, the heat output is such that the temperature ofthe back wall 6 and the side wall 8 will not exceed the maximumspecified by EN60335, but at the same time is sufficient to maintain asignificant volume of water at a fast boil or to fry. At point F, after40 minutes of cooking the user either switches the heater off or to alower power setting. At point G, 20 minutes after the heater was last atfull power level, the user switches the heater back to full power. Thecontrol circuitry maintains the full power (first temperature) level forhalf of twenty minutes, i.e. for 10 minutes, and at point H, after 10minutes at full power, the temperature level reverts to the fallback(second) temperature level.

In practice, the manner in which the time before the heater reverts tofallback temperature level is determined may be established fromexperimental data and could be other than a simple inverseproportionality.

Referring to FIGS. 6 and 7, an electric heater 110 is arranged beneath aglass-ceramic cooking plate 112 in a cooking appliance (not shown indetail). The heater 110 comprises a metal dish 114 having a base layer116 of thermal insulation material, such as microporous thermalinsulation material.

The heater 110 is arranged to provide two concentric heating zones. Amain heating zone 118 is surrounded by an additional heating zone 120,the zones 118, 120 being separated by a dividing wall 122 of thermalinsulation material, a top surface of which contacts a lower surface 124of the glass-ceramic cooking plate 112. A peripheral wall 126 of thermalinsulation material is also provided, having a top surface whichcontacts the lower surface 124 of the glass-ceramic cooking plate 112.

The centrally located main heating zone 118 has at least one heatingelement 128, supported relative to the base layer 116. The additionalheating zone 120 also has at least one heating element 130, supportedrelative to the base layer 116. The heating elements 128, 130 are ofwell known form and may, for example, comprise corrugated metal ribbonelements.

A terminal block 132 is arranged at the edge of the heater 110 and bymeans of which the heating elements 128, 130 are electrically connectedto a power supply 134 for energising.

The heating elements 128, 130 are arranged to be connected so that theheating element 128 can be operated alone, whereby the main heating zone118 is energised alone, for heating a small cooking vessel 136A locatedon an upper surface 138 of the cooking plate 112. The heating element128 can also be operated together with the heating element 130, wherebythe main heating zone 118 is energised together with the additionalheating zone 120, for heating a larger cooking vessel 136B located onthe upper surface 138 of the cooking plate 112.

A temperature sensor 140 is arranged to extend partially across theheater, between the heating elements 128, 130 and the cooking plate 112.The temperature sensor 140 comprises a tube containing a device whichprovides an electrical output as a function of temperature. Such devicemay have an electrical parameter, such as electrical resistance, whichchanges as a function of temperature. In particular, the devicecomprises a platinum resistance temperature detector.

As an alternative to the temperature sensor 140, a temperature sensorcould be provided deposited on, or secured in contact with, the lowersurface 124 of the cooking plate 112.

Control circuitry 142 is provided for the heater 110. Such controlcircuitry comprises a microcontroller 144, which is amicroprocessor-based control unit. An energy regulator 146 is alsoprovided, which has a control knob 148 by means of which a plurality ofuser-selectable energy (power level) settings of the heater 110,including a full power setting, can be achieved in known manner.

Power is supplied to the heater 110 from the power supply 134 by way ofa relay 150, or by way of a solid state switch means, such as a triac.

The temperature sensor 140 is calibrated in association with themicrocontroller 144 to provide an electrical output which is tuned as afunction of temperature of the upper surface 138 of the cooking plate112.

The temperature of the glass-ceramic cooking plate 112 must not exceed acertain level in order to prevent thermal damage to the glass-ceramicmaterial. For optimum cooking performance, it is required to be able toheat up the cooking vessel 136A, 136B and its contents as rapidly aspossible, for example to achieve rapid boiling of the contents of thecooking vessel 136A, 136B. Accordingly, it is desirable for thetemperature of the upper surface 138 of the cooking plate 112, at whichthe temperature sensor 140 operates for controlling the heater 110, tobe as high as permissible. However, as noted previously this must not besuch as to result in an unacceptably high temperature of the cookingplate 112, or an unacceptably high temperature of the back wall 6 orside wall 8, a limit for which is specified in European Safety StandardEN60335.

It has been found that for a heater 110 operated in a free radiationcondition at a full temperature (power) level setting with the mainheating zone 118 energised alone, and controlled by way of thetemperature sensor 140, such conditions can be safely maintained at afirst temperature level for a predetermined initial period without thetemperature of the back wall 6 and side wall 8 exceeding the specifiedlimit. It has been found that such predetermined initial period is fromabout 30 to about 50 minutes and is typically about 40 minutes. It hasalso been found that if, at the end of such predetermined initialperiod, the temperature level of the heater 110 is then reduced from thefirst temperature level to a second temperature level which is betweenabout 75 percent and about 85 percent, preferably about 83 percent, ofthe first temperature level, the temperature of the back wall 6 and sidewall 8 is maintained at a level which does not exceed the specifiedlimit.

If the heater 110 is operated in a free radiation condition at a fulltemperature (power) level setting with the main heating zone 118energised together with the additional heating zone 120, then because ofthe higher resulting power and the larger heated area, the temperatureof the back wall 6 and side wall 8 rises more rapidly and theirspecified temperature limit is reached sooner than when the main heatingzone is energised alone. In this case, the predetermined initial periodwhich can be safely maintained at the first temperature level, beforereducing to the second temperature level, without the temperature of theback wall 6 and side wall 8 exceeding the specified limit, is shorterand is from about 20 to about 40 minutes and is typically about 30minutes. However, under certain circumstances the predetermined initialperiod can be as little as 10 minutes.

The microcontroller 144 is programmed in the factory, during manufactureof the heater 110 and its associated control circuitry, with thenecessary data for the values of the predetermined initial period,according to whether the main heating zone 118 is energised alone ortogether with the additional heating zone 120, and also the value forthe reduced second temperature level. Such programmed data is thereafterautomatically implemented by the microcontroller 144 to safely controlthe heater 110.

The controlling operation is illustrated in FIG. 8, which is a plot ofthe temperature TE in degrees Celsius of the upper surface 138 of thecooking plate 112 (known as the top glass temperature) against time TIin minutes at the full power setting. With the main heating zone 118energised alone, during a pre-set initial period A1 of 40 minutes theheater 110 is operated at a boost power level for a period B of about 7minutes, followed by operation at a normal first temperature (fullpower) level for a further 33 minutes. During the boost period, thetemperature of the upper surface 138 of the cooking plate 112 exceeds600 degrees Celsius and during the remainder of the predeterminedinitial period A1 the temperature of the upper surface 138 of thecooking plate 112 is maintained at around 600 degrees Celsius. Thisenables rapid heating to boiling to take place in the cooking vessel136A. However, during this initial period A1, the temperature of theback wall 6 and the side wall 8 does not exceed the limit specified byEuropean Safety Standard EN60335. At the end of the period A1, themicrocontroller 144 automatically reduces the temperature level of theheater 110 to a lower (second) fallback temperature level which is about75 to 85 percent of the previous full (first) temperature level. Suchreduction, as denoted by reference numeral 152, can be effected in oneor more steps, or continuously. During the subsequent ongoing period C,the temperature of the upper surface 138 of the cooking plate 112 ismaintained at about 500 degrees Celsius and this ensures that the backwall 6 and side wall 8 are maintained at a temperature which does notexceed the specified limit. However, as shown in FIG. 8, the reducedtemperature level is not such as to interfere with a temperature band154, required for frying activities, and a temperature band 156,required for continuous boiling/simmering activities.

When the main heating zone 118 is energised together with the additionalheating zone 120, then because of the higher resulting power andincreased heated area in the heater 110, the temperature of the backwall 6 and side wall 8 rises more rapidly and reaches its specifiedlimit sooner than when the main heating zone 118 is energised alone atthe boost power level followed by the normal full (first) temperaturelevel. In this case a reduced predetermined initial period A2 of about30 minutes is automatically implemented by the microcontroller 144 andat the end of which the temperature level is automatically reduced bythe microcontroller 144 to the lower (second temperature) fallbacklevel, as denoted by reference numeral 152A and shown by the broken lineportion of the graph. This ensures that the specified limit for thetemperature of the back wall 6 and side wall 8 is not exceeded, whileensuring optimised performance of the heater 110.

During normal operation, the heater 110 may be switched off, or to alower power level setting, by a user and then back to full power whilethe temperature of the back wall 6 and side wall 8 is still elevated. Inthis case, the fallback (second) temperature level requires to bere-introduced in a short time compared with the situation when theheater is first energised. In such case, the time at full (first)temperature, originally set to full power, may be reduced by an amountinversely proportional to the time interval since the heater was last atfull power.

Although FIG. 6 shows a heater 110 in which the main heating zone 118 isconcentrically arranged with the additional heating zone 120, otherarrangements are possible. As shown in FIG. 9 a heater 110 may comprisean oval arrangement in which the main heating zone 118, provided withheating element 128, is bordered at one side by the additional heatingzone 120, provided with heating element 130. The heater 110 has aperipheral wall 126 of thermal insulation material and a dividing wall122, also of thermal insulation material.

As shown in FIG. 10, a heater 110 may comprise what is known as an angelarrangement in which the main heating zone 118, provided with heatingelement 128, is bordered on opposite sides by wing-like additionalheating zones 120, provided with heating elements 130. The heater 110has a dividing wall arrangement 122 of thermal insulation material and aperipheral wall arrangement 126, also of thermal insulation material.The heaters 110 of FIGS. 9 and 10 are operated and controlled in thesame way as the heater 110 of FIG. 6.

1. A method of avoiding unacceptably high temperatures of a walladjacent to a cooking appliance comprising: a glass-ceramic plate (12,112) having an upper surface for receiving a cooking vessel (42 136A,136B) and a lower surface; a radiant electric heater (10, 110) arrangedat the lower surface of the glass-ceramic cooking plate (12, 112); andelectronic control apparatus including: a temperature sensor (24, 140)for monitoring temperature at or adjacent to the cooking plate, whichsensor provides an electrical output as a function of temperature; andcontrol means (30, 142) connected to the temperature sensor and to theheater, for controlling energising of the heater from a power supply,the control means being adapted and arranged to energise the heater at aplurality of user selectable power levels including a full power level,wherein the control means (30, 142) is adapted such that, when theheater (10, 110) is energised at the full power level in order to avoidunacceptably high temperatures of a wall adjacent to the cookingappliance, the heater is energised to heat the cooking plate (12, 112)to a first temperature level for a predetermined initial period of 20 to50 minutes and is thereafter energised automatically to heat the cookingplate to a second temperature level, lower than the first temperaturelevel.
 2. A method according to claim 1, wherein during an initial minorproportion of the predetermined initial period the heater (10, 110) isenergised at a boost temperature level, in excess of the firsttemperature level.
 3. A method according to claim 1, wherein the secondtemperature level is between about 75 percent and about 85 percent ofthe first temperature level.
 4. A method according to claim 3, whereinthe second temperature is about 83 percent of the first temperaturelevel.
 5. A method according to claim 1, wherein the length of thepredetermined initial period is dependent on the time elapsed since thecontrol means (30, 142) was last at the full power level.
 6. A methodaccording to claim 5, wherein the length of the predetermined initialperiod is inversely proportional to the time elapsed since the controlmeans (30, 142) was last at the full power level.
 7. A method accordingto claim 1, wherein reduction from the first temperature level to thesecond temperature level is effected in a continuous manner.
 8. A methodaccording to claim 1, wherein reduction from the first temperature levelto the second temperature level is effected in a stepwise manner.
 9. Amethod according to claim 8, wherein reduction from the firsttemperature level to the second temperature level is effected in asingle step.
 10. A method according to claim 8, wherein reduction fromthe first temperature level to the second temperature level is effectedin a plurality of steps.
 11. A method according to claim 1, wherein thecontrol means (30, 142) comprises a microprocessor-based controller (32,144) into which the predetermined initial period and a setting for thesecond temperature level are programmed for automatic implementation.12. A method according to claim 1, wherein the temperature sensor (24,140) provides an electrical output as a function of temperature of theupper surface of the glass-ceramic cooking plate (12, 112).
 13. A methodaccording to claim 1, wherein the temperature sensor (24, 140) comprisesa device whose electrical resistance changes as a function oftemperature.
 14. A method according to claim 13, wherein the temperaturesensor (24, 140) comprises a platinum resistance temperature detector.15. A method according to claim 1, wherein the temperature sensor (24,140) is provided on the lower surface of the glass-ceramic cooking plate(12, 112).
 16. A method according to claim 1, wherein the heater (110)has a main heating zone (118) at least partly surrounded by at least oneadditional heating zone (120), the main heating zone being energisablein a first mode alone and in a second mode together with the at leastone additional heating zone.
 17. A method according to claim 16, whereinthe at least one additional heating zone (120) is arranged substantiallyconcentrically with the main heating zone (118).
 18. A method accordingto claim 17, wherein the at least one additional heating zone (120) isarranged against at least one side of the main heating zone (118).
 19. Amethod according to claim 18, wherein at least one additional heatingzone (120) is arranged at opposite sides of the main heating zone (118).20. A method according to claim 16, wherein the predetermined initialtime is about 20 minutes to about 40 minutes when the main heating zone(118) is energised together with the at least one additional heatingzone (120), and is about 30 minutes to about 50 minutes when the mainheating zone (118) is energised alone.
 21. A method according to claim1, wherein the predetermined initial time is about 20 minutes to about40 minutes.
 22. A method according to claim 1, wherein the temperaturesensor is spaced behind the lower surface of the glass-ceramic cookingplate.